1. Field of the Invention
This invention relates generally to an apparatus and method for adjusting the orientation of a light beam in a raster scan system. More particularly, this invention relates to adjusting the sagittal and tangential orientation of a light beam to correct for differential bow.
2. Description of Related Art
Flying spot scanners (often referred to as raster output scanners or ROS) conventionally have a reflective multifaceted polygon mirror that is rotated about its central axis to repeatedly sweep one or more intensity modulated beams of light across a photosensitive recording medium in a line scanning direction (known as the fast-scan or main direction in the tangential direction) while the recording medium is being advanced in an orthogonal, or "process" direction (known as the slow scan direction in the sagittal direction), such that the beams scan the recording medium in accordance with a raster scanning pattern. Digital printing is performed by serially intensity modulating each of the beams in accordance with the binary sample string, whereby the recording medium is exposed to the image represented by the samples as it is being scanned. Printers that sweep several beams simultaneously are referred to as multi-beam printers. Both ROS and multi-beam printer techniques are illustrated in U.S. Pat. No. 4,474,422 to Kitamura (Issued Oct. 2, 1994), the subject matter of which is incorporated herein by reference.
High speed process color or multi-highlight color xerographic image output terminals require multiple independently addressable raster lines to be printed simultaneously at separate exposure stations. This is called multi-station printing. Conventional architectures for multi-station process color printers use a plurality of separate ROSs, usually four independent ROSs, one for each system color, for example, as illustrated in U.S. Pat. Nos. 4,847,642 (Issued Jul. 11, 1989) and 4,903,067 (Issued Feb. 20, 1990) to Murayama et al., the disclosures of which are incorporated herein by reference.
The problems with these systems are the high cost related to the multiple ROSs, the high cost of producing nearly identical multiple ROSs and associated optics, and the difficulty of registering the system colors.
U.S. Pat. No. 5,243,359 to Tibor Fisli (Issued Sep. 7, 1993), the disclosure of which is incorporated herein by reference, discloses a ROS system suitable for deflecting multiple laser beams in a multi-station printer. FIG. 1 illustrates one embodiment of Fisli's multistation printer 10. For the ease of illustration, the numbering of elements in FIG. 1 of the present application differs from the numbering of elements in the Fisli Patent. A rotating polygon mirror 12 simultaneously deflects a plurality of clustered, dissimilar wavelength laser beams, having their largest divergent angles parallel to one another. The laser beams are subsequently separated by a plurality of optical filters 16, 18 and 20 and are directed onto their associated photoreceptors 22, 24, 26 and 28. Similarly dimensioned spots are obtained on each photoreceptor 22, 24, 26 and 28 by establishing similar optical path lengths for each beam. The laser diodes in U.S. Pat. No. 5,243,359 to Fisli are arranged in the slow scan direction (i.e., sagittally offset). Laser diodes oriented in the slow scan direction must be arranged such that they are packed closely in a direction parallel to the polygon mirror's rotational axis to minimize beam characteristic deviations such as spot size, energy uniformity, bow and linearity. Thus, the laser diodes are kept as closely as possible in the direction parallel to the polygon mirror's rotational axis so that the light beams strike nearly the same portion of the polygon mirror as possible.
U.S. Pat. No. 5,341,158 to James Appel et al. (Issued Aug. 23, 1994), the disclosure of which is incorporated herein by reference, discloses a ROS system in which the laser beams are tangentially offset in the fast scan direction (i.e., separated horizontally) to offset diode spacing constraints of U.S. Pat. No. 5,243,359 to Fisli.
FIG. 2 illustrates a prior art dual spot ROS for a single station printer. Such a single station dual spot ROS can be extended into a multi-station dual spot printer. For illustration purposes, FIG. 2 does not show the laser diodes or the pre-polygon optics. These features are well-known in the art and will be described below in detail. FIG. 2 shows two laser beams 31 and 33 being deflected from a rotating polygon mirror 30 and focused onto a photoreceptor 38 using well-known post-polygon optics 32 and 34 and the folding mirror 36. Such dual spot printers simultaneously print two or more spots on the photoreceptor 38 to increase the speed.
Such systems generally use interlace techniques for imaging the plural light beams on the photoreceptor 38. For example, U.S. Pat. No. 5,233,367 to Curry (Issued Aug. 3, 1993), the disclosure which is incorporated herein by reference, discloses one such multiple beam interlacing scanning system. The present invention is preferably used for dual spot printers such as that shown in FIG. 2, although the invention may also be used with the single spot or multi spot multi-station printer such as described in U.S. Pat. No. 5,243,359 to Fisli.
In single spot rotating polygon based optical systems, bow distortions occur from the accumulation of optic tolerances. Bow itself is the curved line described by the scanned laser beam of the ROS as moves in the fast scan direction. Thus, the bow appears as a displacement of the scan line in the process direction as the line extends in the fast scan direction.
Although multi-beam, laser diode based ROS is viewed as the most powerful technology for high quality xerographic printing, differential scan line bow remains an undesirable side effect. Differential scan line bow rises from the very nature of multi-beam optic systems, where the beams are offset sagittally (i.e., in the slow scan direction). The bow occurs because the magnification varies across each sagittal plane as each of the sagittally offset beams propagate through the optical system.
For example, FIG. 8A illustrates three facets of a polygon mirror 56. In the current facet 100, two scan lines 102, 104 are illustrated to show the respective scanning of each laser beam 31 and 33 in the fast-scan direction. As shown in FIG. 8A, scan line 102 is reflected from an upper half (i.e., above the optical axis 56A) of the current facet 100 while the scan line 104 is reflected from a lower half (i.e., below the optical axis 56A) of the current facet 100.
Depending on the design of the system, the scan line bow can cause the scan lines to move toward each other (barrel distortion), or away from each other (pincushion distortion). In both of these cases, the light sources (lasers) are placed on opposite sides of the optical axis 56A. Therefore, the centers of curvature of the bow scan lines are also on opposite sides of the optical axis as shown in FIG. 9B. If all the light sources are on one side of the optical axis, then all the scan lines will be imaged on the opposite side of the optical axis. Therefore, the centers of curvature of all the bow lines will also lie on the same side of the optical axis. However, each line will be bowed at a different radius or curvature. This is the source of another type of differential bow.
U.S. Patent application Ser. No. 08/174,917 to Tibor Fisli et al., filed Dec. 29, 1993, the disclosure which is incorporated herein by reference, provides for a multi-beam ROS in which the chief exit rays from the last element of the optical system to the photoreceptor are telecentric in the sagittal meridian. By providing telecentric chief exit rays, the multi-beam system becomes both tolerant to pyramidal polygon angular errors and is able to maintain adequately stable, essentially no bow performance over an acceptable depth of focus in the single station xerographic printers. In addition, by closely controlling the overall shape and orientation of the bow, single pass, multi-station systems are able to print with acceptable levels of misregistration between the various images written on the widely separated xerographic stations.